FIG. 1 of the accompanying drawings is a block diagram showing a known technique for measuring RF power.
The unknown RF power to be measured is applied to a detector 1, which converts the RF power into a more easily measurable quantity, such as voltage or temperature (which may in turn be converted to voltage). The voltage representing the RF power is digitised by a digitiser 2. To provide a reading of RF power to the user of the equipment, the output of the digitiser is usually scaled by a numeric correction function in numeric correction block 3, which takes into account the transfer function of the detector, and any fixed offsets which would otherwise give errors at low RF powers.
In a practical implementation of the technique described, the detector block is typically a diode followed by a capacitor. The transfer function of this block, expressed as voltage out divided by RF power in, is a function of frequency, temperature, level and the batch characteristics of the diode. To avoid the user of the equipment having to take these factors into account each time a measurement is made, the detector and digitiser are characterised at initial manufacture using known RF input powers, and the results are retained in the stored numbers block 8. When the user makes a measurement of an unknown RF power, the stored numbers are used by the numeric correction block 3 to scale the output of the digitiser 2, and hence give a corrected measurement.
The process of characterisation at initial manufacture using known RF input powers is called calibration. FIG. 2 of the accompanying drawings is a block diagram showing a known technique for calibration. For simplicity, it will be assumed that the detector is used in a region where the voltage output is proportional to the power input, and that there are no offset errors.
A known RF calibration power RFcalpower produced in power source 9 is applied to the detector 1 at a series of known frequencies Fn covering the range of interest. At each frequency the output of the digitiser gives a value N1Fn, which can be used together with the known RF calibration power to give the transfer function T1Fn of the detector 1 and digitiser 2 at each frequency Fn:
      T          1      ⁢      Fn        =            N              1        ⁢        Fn                    RF      calpower      
This series of numbers T1Fn is retained in the stored numbers block 8.
Referring again to FIG. 1, when the user makes a measurement of an unknown RF power, provided the frequency is known the appropriate stored number can be used by the numeric correction block 3 to scale the output from the digitiser 2, and hence give a reading of the unknown RF power.
This technique takes into account the variability of transfer function with frequency and with the batch characteristics of the diode. It does not however taken into account the variability of transfer function with temperature. If a measurement of an unknown RF power is made at a different temperature from the temperature at the time of calibration, significant errors result.